In the manufacturing of microwave oscillators a planar technique is generally employed, based on the deposition of conductive films in predetermined areas of a dielectric substrate forming a microstrip circuit layout, on which discrete active and/or passive components shall be installed, completing the oscillator circuit. Universally adopted substrates are those presenting low dielectric losses at the highest use frequencies, such as for instance: alumina, quartz, glass, etc., since they do not significantly attenuate the signals crossing the microstrip, making thus easier to obtain the circuit layouts.
The parameter defining dielectric losses in a substrate is the tan. .delta.. also: EQU Q=1/tan. .delta..
High dielectric loss substrates, such as for instance vetronite ones, that is a thin glass fiber laminate soaked with epoxy resins and copper plated on one or both the sides (the standard definition is glass reinforced FR4), are on the contrary generally used in the manufacturing of electronic circuits, operating at considerably lower frequencies compared to microwaves, for instance in the manufacturing of a traditional printed circuits.
For vetronite tan. .delta.=0.025-0.05 whereas for alumina tan..delta..=0,0001.
The comprehension of the boundaries of the application fields of the two substrate typologies is facilitated by the fact that, if one wants for instance to employ the standard glass reinforced FR4 substrates in a microwave environment, these will attenuate the signals too much, preventing in practice the realization of the layouts; on the contrary, when alumina substrates are used at the lowest frequencies, even if possible in itself, it should not be possible to obtain the same advantages as the standard glass reinforced FR4, in terms of easy handling of the product and simple manufacturing process. There is of course the exception, represented by a frequency interval, which includes the first microwaves where the selection between the two kinds of substrates is not so well defined.
In general, we can assert that as the frequency increases, the manufacturing process shall have ever-restricted tolerances since it is necessary a width of the lines or gap among these last lower than 100 .mu.m. This involves a higher definition of line edges available and a better regularity of the same.
Another restriction is due to the installation of components, which in presence of higher frequencies shall have a better accuracy on the positioning of the components themselves to assure the required repeatability in the testing step.
It is necessary to consider the fact that a Microwave VCO used as local oscillator is generally inserted in a phase locked loop, or PLL, for the implementation of the frequency synthesis in the channel selection. The operation of a PLL is known, the emphasis here, is that it includes dividers of the VCO frequency and devices operating at a frequency lower than the microwaves one.
The parameters characterizing a VCO are selected in the design phase since they depend on the application of the VCO itself. In general, it is known that if the Q is high, the phase noise close to the carrier is very good (that is low). However, on the other hand, there is a narrow operation bandwidth (100 MHz). If on the contrary, the Q is low the phase noise is worsened, but a broader operation band is obtained. However, in alumina Q=200 (it depends almost only on the line steel) while in FR4 Q=20-40 with worsening of the phase noise of about 10 dB.
In the engineering practice, the integration of microwave circuits with lower frequency circuits requires the use of sealed mechanical housing to envelope microwave circuits, to avoid the electromagnetic power irradiation in the surrounding space and ensuing malfunctions of the remaining circuits. This involves significant production costs due to:
Accuracy mechanics of the mechanical housing; PA1 The anchoring system of the same to the substrate housing the circuits at lower frequency; PA1 The need to provide coaxial cables fit with terminal connectors for the entering and/or withdrawal of radio frequency signals in/from the microwave modules; PA1 The need to provide appropriate guide/microstrip transitions for the above mentioned signals; PA1 and finally to the particular means supplying the electric power to the microwave modules. PA1 the microwave structure, typically the VCO whose oscillation frequency can be driven up to 18 GHz, which is produced through deposition of conductive films on an alumina substrate; PA1 the lower frequency structure and typically consisting of locking circuits of the VCO and of bias circuits of all the components, including the components through which the VCO itself is made, which is manufactured, for cost saving purposes, using a standard glass reinforced FR4 substrate.
In summary, this is the current state of the technique in the field of the invention, which as noted above, requires two different techniques to produce:
In the following description, this low frequency structure, consisting of the above mentioned locking circuits and of the above mentioned bias circuits is defined in short "control circuits".
A microwave oscillators of the known type, i.e. a VCO based on the deposition of conductive films in predetermined areas of a dielectric substrate presenting low dielectric losses at the highest use frequencies and the other circuit portions on a glass epoxy substrate is disclosed in U.S. Pat. No. 5,187,451.
This Patent in fact discloses a voltage control circuit (VCO) having a resonant circuit of the microstrip line whose line width and line length are minimised.
The VCO operating at an oscillation frequency of 900 MHz, wherein the resonant portion is configured with a high conductivity conductor on a low dielectric loss, e.g. alumina ceramic dielectric substrate, and the other circuit portions may be on a glass epoxy substrate.
In particular FIG. 2(a) of this patent shows a microstrip line 1 of the resonant circuit b constituted by forming a conductor over a low dielectric constant substrate such as an alumina substrate 2 (see column 4 lines 50 to 54).